強震記録から評価した短周期震源スペクトルの地域性

Abstract

We examine the excitation strengths of source spectra in high-frequency range, and discuss their regional variation. Target events are subduction earthquakes off Fukushima Prefecture, intermediate depth earthquakes in the Kanto region, and shallow earthquakes in the southern Kanto region in Japan. The analyzed data are S-wave portions of 672 accelerograms from 40 events observed at 27 stations in the southern Tohoku and the Kanto region. Source spectra are evaluated by simultaneously separating source, propagation path, and local site effects using inversion technique in spectral domain [IWATA and IRIKURA (1986)]. The magnitude of these events ranges from 3.8 to 7.0, and hypocentral distance from 20 to 200km. First of all, the relationship between the seismic moment M0 and the amplitude of source spectra in high-frequency range MHF is examined. MHF is evaluated by taking logarithmic average of acceleration source spectrum from 1 to 10Hz. MHF is nearly proportional to M1/30, which is expected from the scaling of source spectra based on ω-2 model. MHF shows regional variation even if M0 is nearly equal. To quantitatively estimate the amplitude difference in high-frequency range, BRUNE stress drop Δσ [BRUNE (1970, 1971)] is evaluated for each event using M0 and MHF. Average Δσ for shallow earthquakes in the southern Kanto, subduction earthquakes off Fukushima Prefecture, and intermediate depth earthquakes in the Kanto region is about 100 bars, 200 bars, and 370 bars, respectively. Taking into account the relation that MHF is proportional to Δσ2/3, intermediate depth earthquakes excite about 2.4 times larger amplitude than that for shallow earthquakes in high-frequency range. Thus, deeper events generally show large Δσ in comparison with shallower events, suggesting that the focal depth is a key parameter to control the excitation strengths of source spectra in high-frequency range.